1. Field of Invention
One or more embodiments of the present invention relate to a rotation angle detection apparatus for use in fields of industrial machinery, machine tools, and so forth, and in particular, to a rotation angle detection apparatus having a detection unit that can cope with detection targets in a plurality of sizes.
2. Background Art
In response to an increasing demand for machining of extra-large parts in the fields of wind power generation, oil drilling, and the like, numbers of extra-large machine tools have been recently increased. Accordingly, the diameter of a rotation shaft has become larger, and so has that of a rotation angle detection apparatus mounted on the rotation shaft.
FIG. 3 is a cross sectional view of an example of a conventional hollow rotation angle detection apparatus, the structure of which will be outlined below. That is, a slit disk 202 is fixedly mounted on a rotation shaft 201 having a through hole formed therethrough with rotation deflection adjusted, and a regular bright and dark grid is applied to the slit disk 202. Meanwhile, a light emitting device 203 and a light receiving device 204 are securely mounted on the housing 208 so as to together sandwich the slit disk 202. While the slit disk 202 rotates with rotation of the rotation shaft 201, the light receiving device 204 detects parallel light from the light emitting device 203, and sends a voltage level resulting from photoelectric conversion to a processing circuit substrate 205. The processing circuit substrate 205 carries out an interpolation operation or the like, based on the voltage value transmitted, to calculate a rotation position of the rotation shaft 201.
This structure has bearings 206a, 206b and a coupling 207, in which a detection unit is not separated from a detection target. Therefore, the size of a component on the machine side that is inserted through the through hole needs to be smaller than the diameter of the through hole formed in the rotation shaft 201. When a hollow rotation angle detection apparatus having a larger through hole is requested, it is necessary to newly design a structure of the entire hollow rotation angle detection apparatus. Such a structure having integrated detection unit and detection target is described in, for example, Japan Patent Laid-open Application No. 2009-258002.
Meanwhile, there has been a case in which a rotation angle detection apparatus having a detection unit separated from a detection target is used. For example, Japan Patent Laid-open Application No. Hei 11-183201 describes a structure in which an encoder can be separated from a detection unit. Further, Japan Patent Laid-open Application No. 2006-322764 describes a highly accurate rotation angle detection apparatus. FIG. 4 is a perspective view showing one example of such a conventional rotation angle detection apparatus. Specifically, a detection target comprising a detection gear 301 and an absolute position encoding disk 302 is securely mounted on the rotation shaft or the like on the machine side (not shown). The detection gear 301 is a spur gear having 360 teeth and a module of 0.4 with a basic pitch length of about 1.256 mm, in which the inner diameter is φ100 mm. The absolute position encoding disk 302 has irregular cut-offs formed on the external circumference thereof to binarize a code using the cut-offs. The one-bit length is equal to the basic pitch length of the detection gear 301, being about 1.256 mm, and an irregular cyclic code is given such that each of the codes for respective sets of nine successive bits read is unique.
Meanwhile, a detection unit 303, fixedly mounted on a non-rotating member, such as a flange, on the machine side (not shown), includes eight magnetoresistive elements for detecting a magnetic flux density that will change due to the uneven part on the detection gear 301, and nine magnetoresistive elements for detecting a magnetic flux density that will change due to the cut-offs on the absolute position encoding disk 302.
The eight magnetoresistive elements for detecting the uneven part of the detection gear 301 are placed so that orthogonal two-phase signals are obtained. That is, relative to a first element used as a reference, second, third, fourth, fifth, sixth, seventh, and eighth elements are arranged displaced in the measurement axial direction by ¼ pitches (about 0.314 mm), ½ pitches, ¾ pitches, 1 pitch, 5/4 pitches, 3/2 pitches, and 7/4 pitches, respectively, in which the first and fifth elements detect a sine positive phase, the second and sixth elements detect a cosine positive phase, the third and seventh elements detect a sine negative phase, and the fourth and eighth elements detect a cosine negative phase. Meanwhile, the nine magnetoresistive elements for detecting the cut-offs of the absolute position encoding disk 302 are arranged so that a signal that binarizes presence or absence of a cut-off can be obtained. That is, the nine elements are arranged apart from each other by one pitch length so that a code for nine successive bits is read.
It is assumed here that the through hole of the rotation angle detection apparatus is desired to be enlarged from φ100 mm to φ150 mm. FIG. 5 is a perspective view of a rotation angle detection apparatus having a through hole of φ150 mm. The detection gear 401 is a spur gear having 500 teeth and a module of 0.4 with a basic pitch length of about 1.256 mm long, in which the inner diameter thereof is φ150 mm. The absolute position encoding disk 402 has irregular cut-offs formed on the external circumference thereof to binarize a code using the cut-offs, similar to the example shown in FIG. 4. The one-bit length is equal to the basic pitch length of the detection gear 401, namely, about 1.256 mm. An irregular cyclic code is given such that each of the codes for respective sets of nine successive bits is unique.
Meanwhile, a structure identical to that of the detection unit 303, shown in FIG. 4, is used for the detection unit 303. As the detection gears 301 and 401 have an identical basic pitch length, it is unnecessary to change the positions of the eight magnetoresistive elements for detecting the uneven parts of the gear 401. Similarly, as the absolute position encoding disks 302 and 402 have the same basic pitch length, it is unnecessary to change the positions of the nine magnetoresistive elements. Machining of gears having an identical basic pitch length (a product of a module and the Ludolphian number) but a different number of teeth is achievable using the same tool (a hub cutter). Therefore, designing, a process programming period, and so forth, for manufacturing a new detection gear is not much of a problem. That is, as described above, as long as a detection target is separated from a detection unit and the basic pitch of detection targets are the same despite being different sizes, the detection unit can cope with detection targets in various sizes, so that a request on a threshold hole size made by a machine side can be relatively easily coped with.
FIG. 6 is a diagram showing a schematic structure of a circuit for processing a signal obtained by the detection unit shown in FIG. 5. A resistance change level (a voltage level after conversion) generated by the magnetoresistive element 101a to 101h for detecting a magnetic flux density that will change due to the uneven part of the detection gear 401 is sent to the differential amplifier 102a, 102b so that signals in the same phase are connected to each other and amplified by utilizing a difference in polarity, and thereafter, digitalized in the analogue/digital converter 103a, 103b. The digitalized two-phase signal Sdo, Cdo is subjected to subtraction of an offset correction value Sofs, Cofs therefrom by the subtractor 105a, 105b, in which the offset correction values Sofs, Cofs are stored in advance in the memory 104a. The two-phase signal Sd, Cd with an offset component removed therefrom is converted into a tangent signal Tan_d by the divider 106, and is then subjected to an arctangent operation by the operation unit 107a to thereby obtain an absolute position θp within the basic pitch.
A resistance change level (a voltage level after conversion) generated by the magnetoresistive element 101i to 101q (for brevity, the nine elements are not all shown) for detecting a magnetic flux density that will change due to the cut-offs of the absolute position encoding disk 402 is amplified by the amplifier 102c to 102k before being digitalized by the analogue/digital converter 103c to 103k into a digital signal A1do to A9do. The digital signal A1do to A9do is subjected to subtraction therefrom of a threshold level (an offset correction value) A1ofs to A9ofs for binary determination by the subtractor 105c to 105k to be a positive/negative binarized signal A1d to A9d, in which the threshold levels A1ofs to A9ofs are stored in advance in the memory 104b. 
The positive/negative binarized signal A1d to A9d is subjected to absolute position processing in the operation unit 107d together with the value “500” indicative of the data size (the number of teeth of the detection gear 401 or the maximum code length of the absolute position code of the absolute position encoding disk 402) on a detection target, obtained from the control unit 108 connected to the rotation angle detection apparatus, to thereby obtain a position θa indicative of a position within one rotation divided into 500 parts. Thereafter, the absolute position θp within the basic pitch and the position θa indicating a position within one rotation divided into 500 parts are subjected to digit adjustment (a bonding process) by the CPU 107c to thereby obtain an in-rotation absolute position θ.
A rotation angle detection apparatus having the above described detection unit can relatively easily cope with various through hole sizes, that is, various kinds of gear sizes (the number of teeth). However, although various detection gears 301, 401 or absolute position encoding disks 302, 402 can be coped with, the distances from the magnetoresistive elements 101a to 101q, placed relatively far from the center of the detection unit, to a detection target are changed significantly as the size of the detection target is changed.
Regarding the example of the magnetoresistive elements 101a to 101q, shown in FIG. 6, of the magnetoresistive elements 101a to 101h for detecting a magnetic flux density that will change due to the uneven part of the detection gear 401, the distances from the magnetoresistive elements 101a, 101b, 101g, and 101h, placed relatively close to ends in the measurement axial direction, to a detection target are changed significantly as the size of the detection gear 401 is changed, and the middle level in change of a signal, that is, an offset level, is also changed. This leads to a case in which the offset correction values Sofs, Cofs stored in the memory 104a are not optimum.
Similarly, on the absolute position code side, the distances from the magnetoresistive elements 101i to 101q, placed relatively far from the central position, to a detection target are changed significantly as the size of the absolute position encoding disk 402 is changed. Accordingly, the middle level in change of a signal, that is, a threshold level (an offset level), is also changed. This leads to a case in which the threshold level (an offset correction value) stored in the memory 104b is not optimum.